276 SEX IN MICROORGANISMS 



ferent ploidy levels, either in subnuclei or in the macronucleus as a 

 whole. 



An evaluation of these alternative formulations is complicated 

 by the fact that the macronucleus undoubtedly contains many sets of 

 nuclear genes (Sonneborn, 1947), and the manner in which these are 

 organized in the macronucleus is not well understood. Nevertheless, 

 certain conclusions seem warranted. 



1. The polyploid interpretation in any simple form is untenable. 

 This conclusion is derived from several different lines of evidence. 

 (a) The experimental production of cells with different ploidy levels 

 did not give the results expected on this interpretation (Sonneborn, 

 1953). (b) The mating type system in Tetrahyviena pyrijonms, 

 which shows a remarkable series of parallels to that in P. aiirelia, 

 involves at least seven different mating types (Nannev and Caughey, 

 1953). Although two ploidy levels, characteristic for different mat- 

 ing types, are conceivable, the necessity for postulating seven or more 

 different ploidy levels becomes too great a burden for the hypothesis 

 to sustain. Certainly ploidy alone is not sufficient to account for 

 caryonidal inheritance of mating types in Tetrahymena. {c) It has 

 not been possible to demonstrate any significant differences in the 

 deoxyribose nucleic acid content of cells of different mating types 

 in variety 4 of P. aiirelia (Guthe, Tefankjian, and Nanney, unpub- 

 lished). 



2. An aneuploid interpretation that postulates the loss of a chro- 

 mosome while the macronuclear anlagen are still diploid cannot be 

 supported. This is shown by the fact that all heterozygotes studied, 

 including specifically the Kk heterozygotes (Sonneborn, 1947), have 

 shown the dominant phenotype. If the eliminated chromosome con- 

 tained the dominant allele (as it should at least in some cases), the 

 recessive allele would have been manifested. 



3. An aneuploid interpretation that holds that chromosome dou- 

 bling occurs either in the diploid stage or later could account for the 

 observations. Similarly, the loss of certain chromosomes after the 

 macronucleus has developed could account for the observations. On 

 the other hand, neither of these interpretations can be tested with 

 present technics and the mechanism whereby some one type of 

 chromosome could be regularly, quantitatively and specifically in- 

 creased or eliminated is obscure. Hence, the gene-dosage hypothesis 

 must be considered to have lost its chief utility. 



